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The suspects were detained at a residential address on Tayozhnaya Street.

An FSB statement said: "After committing terrorist attacks, the detained were planning to go to Syria to participate in military actions on side of Islamic State."

One suspect Ravshan Karimov, a married father of four from from Kyrgyzstan, lived in a building close to where the bomb-making materials were found.

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A large amount of cash was also found

A neighbour said: "We would have never guessed. They seemed an ordinary family, he was always working, his wife was busy with kids.

"They were not arguing, nor making noise."

Russia has faced a number of attacks by so-called Black Widows, specially trained women Muslim extremist suicide bombers whose husbands were killed by the security forces in war-ravaged Chechnya and neighbouring areas in the TransCaucasus.

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Russia have been targeted repeatedly by female suicide bombers

In 2013, an explosion in a bus perpetrated by Naida Asiyalova, 30, killed six and injured 33. The same year in Dagestan, a killer identified as Madina Alieva, 25, killed three and wounded 20.

Three years earlier, in March 2010, two explosions on the Moscow metro killed 40 and wounded 100.

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Russian authorities think attacks may have been planned for the subway

The "Black Widows" were identified as Janet Abdurakhmanova, 17 and Mariam Sharipova, 28.